jmc

There are $\left(\genfrac{}{}{0ex}{}{n}{k}\right)=\frac{n!}{k!(n-k)!}$ ways to choose $k$ objects from a group of $n$ distinct objects, so $\left(\genfrac{}{}{0ex}{}{6}{4}\right)=\frac{6!}{4!2!}=\frac{6\cdot 5}{2}=\boxed{15}$ four-member committees may be formed from a group of six students.